As known, bridges provide a structural pathway over depressions or obstacles to facilitate pedestrian and or vehicular and or railway travel over the obstacle. By far the bridges with the highest to frequency of use are the roadway bridges of the interstate and state highway systems, which have tens of thousands of vehicles crossing them each day. They are a critical part of the country's transportation infrastructure, and structural failure of a bridge creates a serious public safety risk as well as disruption of commerce.
State and Federal Departments of Transportation regulations require bridge safety inspections. These inspections have principally involved visual inspection of the bridge structure for signs of deterioration, such as corrosion, erosion, and signs of stress fatigue. However, the increased number of catastrophic bridge failures in the recent decade has prompted many state DOTs to begin conducting dynamic load testing of bridges to better evaluate their structural integrity.
The State of Alabama, for one, performs load tests on bridges which have been designated "load limited" due to actual traffic volume being larger than the theoretically allowable volume for the particular bridge or the bridge design. This testing program consists of instrumenting the bridge structure with strain gauges and/or linear variable displacement transformers, which are connected to a data acquisition system. Known static and dynamic loads are applied to the bridge using weighted DOT trucks which perform a sequence of maneuvers simulating actual traffic loads. The measured stresses (e.g. flexure of key structural members) are compared to the bridge's de sign values and a determination of structural integrity is made. If the bridge is unsafe it is closed, but in most instances the actual stress are far below design criteria due to conservative design calculations and the fact that materials used in the construction of the bridge often exceed specification. If, however, the bridge conditions change, such as an increase in traffic frequency, a higher proportion of heavy load vehicles, or more sever environmental changes, the operational life expectancy of the bridge changes accordingly.
Present methods of measuring bridge load capacity require extensive contact sensor or gage placement, long wire/cable routing, scaffolding, or cherry picker use to gain accessibility. These are labor intensive tasks which require special safety precautions to prevent worker injury. As a result these inspections can only be performed periodically. However, post analysis of structural failures indicates that bridges deteriorate in an unpredictable manner. In some cases structural deterioration may go unnoticed despite load tests due in part to the limited dynamics which may be observed with the staged loads. New methods and equipment are required to quantitatively measure the overall use (or real time) conditions of a bridge, and to correlate measured data to the bridge's condition of deterioration or change in load capacity with time.